Carbazole/iridium dendrimer side-chain phosphorescent copolymers for efficient light emitting devices

Abstract

Dendrimers have proved to be successful materials for solution-processable phosphorescent organic light emitting diodes (OLEDs). Recently we have been investigating poly(dendrimer)s with improved viscosity for ink-jet printing applications. In this work we present three phosphorescent side-chain copolymers comprised of a poly(styrene) backbone, carbazole charge transporting units, and iridium(III) complexes containing no dendrons (simple complex), a single dendron attached to two of the ligands, or doubly dendronised complexes with two dendrons added to the ligands. In addition to their charge transporting capability the copolymer structure incorporating carbazole units can also prevent aggregation of the chromophores, as evidenced by there being only one emissive environment in time-resolved luminescence. We report high external quantum efficiencies of 11.0% (37.3 cd A À1 ) at 100 cd m À2 and 8.3 V for devices made with the neat copolymers. The best performance is achieved by blending the doubly dendronised copolymer with 50 wt% of 4,4 0 -bis(N-dicarbazolyl)biphenyl (CBP) which results in 14.7% EQE (48.3 cd A À1 ) at 100 cd m À2 and 9.3V.

A detailed investigation of light-harvesting efficiency of blue color emitting divergent iridium dendrimers with peripheral phenylcarbazole units

“…22,23 Dendrimers have proved to be successful materials for solution-processible phosphorescent organic light emitting diodes (OLEDs). [24][25][26][27][28][29][30][31][32][33][34][35] Divergent iridium dendrimers are classified into two types: one with engagement of rigid branching units such as alkene and alkyne connectors, and the other with flexible branches, such as connecting with alkyl or alkoxy groups. The flexible branches are generally used in order to secure the synthetic versatility.…”

Recent Advances in Polymer Organic Light-Emitting Diodes (PLED) Using Non-conjugated Polymers as the Emitting Layer and Contrasting Them with Conjugated Counterparts

“…For example, the highest EQE obtained by DDIr-P1-co-VK copolymer was 14.7%, 30 nearly 50% higher than that of DDIr-P1 homopolymer (EQE: 9.2%). 31 One likely reason is that the 1,3-bis(2-ethylhexyloxyphenyl)phenyl dendron was neither a p-type nor a n-type moiety, and; hence, charge transport in the emitting layer was hampered.…”

“…Among the phosphorescent PLEDs, the best performing non-conjugated polymer (DDIr-P1-co-VK) exhibited an EQE of 14.7% with a green emission (k EL at 545 nm) 30 while the conjugated counterpart (P-R-3) exhibited an EQE of 16.1% with a red emission (k EL at 640 nm). 118 On the other hand, the best non-conjugated TADF polymer (TADF-P2) offered a green PLED (k EL at 533 nm) with an EQE of 20.1% 21, whereas the conjugated counterpart (PCzDP-10) gave a PLED with a blue emission at 496 nm and an EQE of 16.1%.…”

“…Levell et al then reported a follow-up study by replacing insulating styrene units with electroactive carbazole functionality to boost the hole mobility of the polymers, 30 given poly(N-vinylcarbazole) is a widely employed hole-transporting material. 11 They prepared a series of random copolymers (Ir-P1-co-VK, DIr-P1-co-VK and DDIr-P1-co-VK, Fig.…”

“…113 In addition to being unable to detect trace elements like halogens and transition metals, CHN analysis has a serious limitation, when it comes to random copolymers whose exact monomer compositions cannot be confirmed due to the broadness and overlap of aromatic peaks in the 1 H NMR spectra. 30,45,116 Halogenated impurities have been shown to be detrimental to the PLED device lifetime due to radical chain type degradation. 137 In addition, palladium black formed during polycondensation can be hard to remove completely.…”